Remove CSP and async support

[chklauser]

Summary

• remove channels, channel selection, call\\async, and async_seq from the runtime
• remove their compiler macro registration, standard-library exports, and PSR CSP helpers
• remove CSP-specific tests and documentation references
• restore select to its non-CSP meaning as the existing alias for map, with a regression test

Validation

• dotnet test --no-restore --tl:off (1,432 total; 1,429 passed; 3 skipped)
• dotnet csharpier check .